(1) Field of the Invention
The present invention relates to a cold cathode serving as a source of electron emission, and more particularly to a field emission type cold cathode for emitting electrons from tips of cones.
(2) Description of the Related Art
There is a report that, by employing a micro-machining technique used for producing fine structures and with an LSI fabrication technique, a field emission type cold cathode can be produced on silicon wafers (reported by C. A. Spindt et al in Journal of Applied Physics, Vol. 39, pp. 3504-3505, 1968). The reported process of production and the structure of the cathode are briefly explained with reference to FIGS. 1A-1D which are diagrammatic sectional views of the structure under fabrication. First, an insulating layer 4 having a thickness of 1.mu.m and a gate electrode 5 of molybdenum are formed on a single crystal silicon. A cavity 6 having a diameter of about 1.5 .mu. m is then formed such that it penetrates the insulating layer 4 and the gate electrode 5 (FIG. 1A). Then, by rotating the substrate 1 about a normal line which extends through the center of the substrate 1, a sacrificial layer 12 of aluminum (Al) is formed by a vacuum deposition process in a direction 70.degree. from the normal line on the gate electrode 5 and also on a portion of the side surface of the cavity 6 (FIG. 1B).
Then, by rotating the substrate 1 about the normal line extending through the center of the substrate 1, a high-melting point metal, for instance, molybdenum (Mo), is deposited by the vacuum deposition process in the direction of the normal line. With progressive deposition of a high-melting point metal layer 13 of Mo on the gate electrode 5, the top opening of the high-melting point metal layer 13 formed on the cavity 6 is progressively reduced because Mo is also deposited on the side surface of the top opening of the high-melting point metal layer 13. During this time, Mo is also deposited on the bottom surface of the cavity 6, and the area of this deposition layer is progressively reduced in conjunction with the progressive reduction of the top opening of the high-melting point metal layer 13. When Mo is deposited until the top opening of the high-melting metal layer 13 is completely closed, a deposit formed on the bottom surface of the cavity has a conically shape (hereinafter referred to as emitter cone) 7 (FIG. 1C). After the high-melting point metal layer 13 has been formed in this way, it is removed by the lift-off process. This is performed by dissolving the sacrificial layer 12 by dipping the wafer in phosphoric acid or other such weak acid. In this way, a fine field emission type cold cathode is obtained (FIG. 1D). By applying a voltage of several tens of volts to 200 V between the substrate 1 and the gate electrode 5 for the gate electrode 5 to be a positive potential, an electric field of 10.sup.7 V/cm is generated at the tip of the emitter cone 7 and electrons are emitted from the tip of the emitter cone 7.
The current development is that emission currents of 100.mu.A and greater per emitter cone have been observed, and various modified applications have been proposed. For example, attempts have been made to produce a fine structure triode switching element using the above element as a source of electrons, and a display panel using a number of above elements in a matrix array which serves as a flat light emission source for the light emission from a phosphor. In the case of a field emission type cold cathode with an array of elements, there is a technique of providing a resistive layer between the emitter cones 7 and the substrate 1 to make the operation characteristics of these elements uniform. By the insertion of a resistor, a voltage drop due to the resistor becomes large in an element that emits a large amount of electrons. In other words, with an element that emits a large amount of electrons, since the voltage at the tip of the emitter cones 7 becomes low thereby reducing the electrons to be emitted therefrom, it is possible to make the operation of the elements uniform. Also, the provision of the resistor between the emitter cones 7 and the substrate 1 has an effect of suppressing the variations in the currents emitted from the emitter cones 7.
Methods of connecting resistors to the emitter cones 7 are proposed in Japanese Patent Application Kokai Publication No. Hei 5-47296, Japanese Patent Application Kokai Publication No. Hei 5-36345 and Japanese Patent Application Kokai Publication No. Hei 4-292831. The arrangement proposed in the Japanese Patent Application Kokai Publication No. Hei 5-47296 is shown in FIG. 2. As shown, in this case, a conductive layer 3 is formed when forming the emitter cones, so that the conductive layer 3 exists as a lower portion of each emitter cone 7. The field emission type cold cathode proposed in the Japanese Patent Application Kokai Publication No. Hei 5-36345 is shown in FIG. 3. Here, each emitter cone 7 is formed from part of a silicon substrate 1 by selectively oxidizing and etching the substrate. This cold cathode, as is the case with FIG. 2 cold cathode, has a conductive layer 3 formed as a lower portion of each emitter cone 7. The arrangement proposed in the Japanese Patent Application Kokai Publication No. Hei 4-292831 is shown in FIG. 4. Here, the field emission type cold cathode is obtained by forming a power supply line layer 11 and a uniform conductive layer 3 on an insulating substrate 10 and forming emitter cones 7 in areas in which the conductive layer 3 and insulating substrate 10 are in contact with each other.
Concerning the prior art field emission type cold cathodes in which a resistive layer is provided between the substrate and the emitter, it has been proposed to form a resistive region by such means as ion implantation in a conductive substrate. Such a process, however, has a drawback that it is difficult to obtain, with satisfactory controllability and reproducibility, a resistance of 100 K.OMEGA. to 10 M.OMEGA. that is necessary for the resistive region because of limitations imposed in forming the resistive region on the constructive substrate. Another drawback is that the formation of a resistive layer occupying a required large area results in a reduction of the element integration density in correspondence to the occupied area.
A further drawback is that, where a continuous resistive layer is provided uniformly for a plurality of elements as a group, the extent of the voltage drop effect obtained by the resistive layer at a central portion is different from that obtained at an edge portion within the group, resulting in non-uniformity of loads on the elements.